1. Field of the Invention
The present invention relates to a fiber amplifier and, more particularly, an erbium (Er)-doped fiber amplifier capable of minimizing band crosstalk.
2. Description of the Related Art
Recently, the demand for a higher transmission bandwidth is increasing in an exponential fashion in the Wavelength-Division-Multiplexing (WDM) communication system. As such, many research efforts have been focused on a wideband transmission system using both the Conventional Band (C-band) and Long Band (L-band). The C-band transmission occurs at a wavelength band ranging from 1530 to 1560 nm, whereas the L-band transmission occurs at a wavelength band ranging from 1570 to 1610 nm. However, these wavelength band transmissions can vary according to the design implementation or characteristics of the Er-dope fibers.
An Erbium Doped Fiber Amplifier (EDFA) is widely used to amplify optical signals in most optical transmission systems. The Er-doped fiber amplifier is restricted in bandwidth, for example, the C-band and L-band have a bandwidth of about 30 nm, respectively. In contrast, a Raman amplifier or a Tellurite-based Er-doped fiber amplifier has a wider amplification bandwidth, such that both the C- and L-bands can be amplified at the same time. However, the Raman amplifier requires high pumping power. Meanwhile, the Tellurite-based Er-doped fiber amplifier is a new technology that has not been verified to be reliable. As such, the utilization of the wideband Er-doped fiber amplifiers has been improved to amplify both the C- and L-bands simultaneously using a conventional Silica-based Er-doped fiber amplifier. To this end, a C-band Er-doped fiber amplifier is typically combined in parallel with an L-band Er-doped fiber amplifier, as shown in FIG. 1.
Referring to FIG. 1, a conventional Er-doped fiber amplifier 10 comprises first and second Er-doped fiber amplifiers 118 and 124 that are connected in parallel to amplify the propagating optical signals, respectively. The first and second Er-doped fiber amplifiers 118 and 124 function to amplify optical signals in C- and L-bands, respectively. A C/L splitter 112 is provided at one end to split the optical signal inputs, and a C/L combiner 130 is provided at the other end to combine the split optical signals. Furthermore, the first and second isolators 110 and 132 are provided to the downstream of an input terminal and the upstream of an output terminal, respectively, to prevent the optical signals from propagating backward.
The C-band optical signals that are split through the C/L splitter 112 are amplified via the first Er-doped fiber 118 with pumped light from a 980 nm pumping laser diode 116 connected thereto via a pump combiner 114. The pumped light energizes Er ions from the ground state and the stimulated emission of the energized Er ions amplifies the C-band optical signals as they pass through the first Er-doped fiber 118. The amplified optical signals are combined in the C/L combiner 130.
Meanwhile, the L-band optical signals that are split via the C/L splitter 112 are amplified using the second Er-doped fiber 124 with pumped light from a forward 980 nm pumping laser diode 122 and a backward 1480 nm laser diode 128 connected to the second Er-doped fiber 124, respectively, via pump combiners 120 and 126. The pumped light energizes Er ions from the ground state and then the stimulated emission of the energized Er ions amplifies the L-band optical signals as they pass through the second Er-doped fiber 124. The amplified L-band optical signals are combined with the C-band optical signals in the C/L combiner 130 and the combined optical signals propagate to the output terminal.
However, the conventional Er-doped fiber amplifier has drawbacks in that it requires a large number of optical components as the respective Er-doped fiber amplifiers for the C- and L-bands are separately manufactured and connected in parallel configuration. As such, the manufacturing cost is high for the conventional amplifier. Moreover, the C/L splitter 112 with a large amount of insertion loss of approximately 0.7 dB is arranged in the upstream of the first and second Er-doped fibers 118 and 124 so as to worsen the noise coefficient of the fiber amplifier disadvantageously.